Wear-resistant stainless cutting element of an electric shaver, electric shaver, and method of producing such a cutting element

ABSTRACT

Disclosed is a cutting element as used in an electric shaver. Said element is manufactured from maraging or precipitation-hardenable stainless steel with a surface hardened by plasma nitriding. The cutting element is plasma nitriding hardened on all surfaces of the blade, and a plasma nitriding hardened layer comprises a surface top layer of steel supersaturated with nitrogen and a diffusion layer adjoining the top layer with a hardness ranging from the hardness of the top layer to the hardness of the steel before hardening by means of plasma nitriding. Also disclosed is an electric shaver comprising at least one of the above cutting elements, as well as a method of manufacturing a cutting element.

The invention relates to a cutting element as used in an electric shaver(also known as additive type shavers), manufactured from maraging orprecipitation-hardenable stainless steel or austenitic stainless steelwith a surface hardened by plasma nitriding. The invention also relatesto an electric shaver provided with such a cutting element and a methodof manufacturing a cutting element.

Since the introduction of the Philips Coolskin® additive shaver, anelectric shaver that can be used with water, and which uses an additivereleased during operation, it has been found that the stainless steelouter cutting element shows unexpected high wear, leading to customercomplaints. It is therefore necessary to provide a more wear-resistantblade, that is, a blade made from a harder material. On the other hand,the blade should not just be sufficiently hard, but also verycorrosion-resistant. Corrosion resistance is less of an issue withconventional shavers, but because of the concept of the CoolskinPhilishave the blade is in much closer contact with moisture. Althoughabove especially mentioned in relation to the Coolskin Philishave typeof shaver it will be clear that improvement of the corrosion resistanceof a cutting element of another type of shaver is also advantageous. Atpresent the material used for the manufacture of these cutting elementsis stainless maraging steel. This is a steel type with good corrosionresistance qualities but with moderate wear resistance. To increase thehardness, the material is hardened by conventional heat treatmenttechniques.

Steels that are very well corrosion-resistant are in most casesdifficult to harden by heat treatment and have poor tribologicalproperties, with consequent inadequate wear characteristics for the usein the additive shaver mentioned above. The wear of the outer blade isnot just caused by contact with the moving (e.g. rotating or linearmoving) blade inside the shaver head but also through the contact withthe skin and hairs, which can be very tough, especially as stubble.

The hardness can be further improved by plasma nitriding, as has beendemonstrated by patent documents U.S. Pat. No. 5,851,313 and DE10039169. In this context also the Japanese document JP 60162766 isrelevant. This document discloses the nitriding of a stainless steel ornickel cutting element for achieving a better durability and a lessersliding load, for example a better smoothness. According the Japanesedocument, only the outside of the blade is hardened at one side by asimple method.

The object of the invention is to provide a means for manufacturingcutting elements that are both very well corrosion-proof and verywear-resistant on all sides.

This object is achieved by a cutting element, as used in an electricshaver, manufactured from maraging or precipitation-hardenable stainlesssteel or austenitic stainless steel with a surface hardened by plasmanitriding, characterized in that the cutting element is hardened byplasma nitriding on all surfaces of the blade, and a plasma nitridinghardened layer consist of a surface compound top layer of steelsupersaturated with nitrogen and a diffusion layer adjoining the toplayer with a hardness ranging from the hardness of the top layer to thehardness of the steel before hardening by means of plasma nitriding,said surface compound layer preferably having a hardness of at least1300 HV, and in the case of austenitic stainless steel at least 1100 HV.The solution provided by the present invention is to comprehensivelyplasma nitride the cutting element, that is on all sides, de factogiving the entire blade an outer layer of hardened material, making itbetter wear-resistant on all sides where wear could possibly occur. Theadvantage of the presence of the diffusion layer is that it additionallystrengthens the base material and supports the load-bearing capacity ofthe compound layer. With a cutting element is meant an individuallyworking shaver blade or a shaver blade that works in cooperation withanother shaver blade. Such a construction of cooperating shaver bladesmay be found, for example, in a shaver with an internal rotating cuttingelement that is surrounded by an external counter cutting element (cap)that has a stationary position. Another construction of cooperatingshaver blades can for instance be found in a shaver with an internalreciprocating (e.g. linear) moving cutting element has is surrounded byan external counter cutting element (cap) that has a stationaryposition. Both the internal rotating moving element and the externalstationary counter cutting element are denoted cutting elements in thisdocument.

In a preferred embodiment, the cutting element has a hardenedsupersaturated top layer with a thickness that ranges from 5 μm to 25 μmand diffusion layer with a thickness that ranges from 5 μm to 20 μm. Inanother preferred embodiment, the hardness of the hardenedsupersaturated top layer is at least 1300 HV, and in the case ofaustenitic stainless steel at least 1100 HV. The cutting element may bedesigned for use in a shaver of the dry shaver type or for use in ashaver of the additive shaver type, for use in a rotating shaver type, areciprocating shaver or a shaver showing another type of relativemovement.

The invention also relates to an electric shaver provided with a cuttingelement as disclosed. Such a shaver has the advantages as mentionedabove in relation to the cutting element according the invention. Onceagain is noted that the electric shaver according the present inventionis not restricted to a specific type of electric shaver; all types ofelectric shavers can be provided with the cutting element as disclosed.

The invention also provides a method of manufacturing a cutting element,characterized in that a cutting element is formed of stainless maragingsteel, whereupon the cutting element is hardened on all surfaces bymeans of plasma nitriding to a hardness of the top layer of at least1300 HV. The method according the invention enables the manufacture ofshaver components from non-hardened (austenitic) stainless steel, whichcomponents are hardened later in the production process by inward growthof a hard and wear-resistant compound top layer, thus simplifying theproduction process. The non-hardened stainless steel can be processedrelatively easily. Another problem encountered in the process ofmanufacturing the cutting elements for the Coolskin® type shaveraccording to the prior art is that the hardenable steel used in theprior art for producing shaver heads (for example, Sandvik 1RK91maraging steel used until now for the production of shaver heads) canonly be bought from one source. This undesirable situation, both from alogistical and a commercial point of view, is now solved according thepresent invention as the method according the invention makes itpossible to use hitherto unsuitable (and relatively inexpensive) typesof steel for producing the cutting element according the invention.Nitriding parameters may be: temperature 300° C. to 500° C., processtime of 5 to 40 hours, nitriding pressure 250 Pa to 550 Pa, and a pulsedplasma process.

The present invention will be elucidated below with reference to theannexed drawings, in which:

FIG. 1 is a microscopic view of nitrided 1RK91 maraging steel,

FIG. 2 shows a diffusion profile in NPR+hardened 1RK91 steel,

FIG. 3 is a cross-section of a hardened lamella,

FIG. 4 is a schematic lengthwise section of a lamella of stainlessmaraging steel,

FIG. 5 is a schematic lengthwise section of a lamella of austeniticstainless steel and

FIG. 6 is a schematic section of a lamella, showing compound layers anddiffusion zones.

The present invention provides a method for the manufacture of a cuttingelement by hardening stainless maraging steel, which method consists inplasma nitriding of the manufactured cutting element in such a way thatthe entire surface of the blade consists of a compounds layer ofsupersaturated steel, below which lies a diffusion layer in which thenitrogen from the compound layer has diffused into the steel, creating ahardness gradient. A cutting element manufactured according to theinvention has a hardness of around 1500 HV, which is exceptionally highin relation to the prior art.

Besides stainless martensitic maraging steel, austenitic maraging steelis also suitable for use in the manufacture of a shaver head accordingto the invention. Actually, austenitic stainless steel is preferredbecause of its greater corrosion resistance compared with martensiticsteel; also it is more widely available. With plasma nitriding it can bemade sufficiently hard-wearing, and if the nitriding temperature is keptbelow 450° C. the anticorrosive properties of the austenitic steel arenot adversely affected.

During the nitriding process, nitrogen penetrates and diffuses into thebase material from the outside inwards. In the so-called compound layerthe hardness is quite even and the metal structure is supersaturatedwith nitrogen. The thickness of this layer depends on the duration ofthe nitriding process. Underneath this layer lies the diffusion zone, inwhich nitrogen diffuses into the base material, the hardness of whichdecreases with depth. FIGS. 1 and 2 illustrate this phenomenon.

Maraging steel and precipitation-hardenable stainless steel can undergoa precipitation hardening step prior to or together with the plasmanitriding step according to the invention.

As is shown in FIG. 6, the diffusion zones in a lamella of a shaver headaccording to the invention are nearly meeting or overlapping. Thehardness of the outer surface depends on the material used. FIG. 4 showsa hardness of 1500 HV for the compound layer and an average hardness ofthe diffusion layer of 500 HV. For an austenitic stainless steel, thedata are 1400 to 1600 HV and above 200 HV, respectively, as is shown inFIG. 5. These values are unusual and hitherto unknown in the state ofthe art. Since the diffusion zones underneath the compound layers arenearly meeting or even overlapping, the mechanical strength of thelamellac is considerably increased. Hardening of the metal is usuallyachieved at the cost of toughness. In other words, it becomes morebrittle. If the blade were uniformly hardened through and through to ahardness of 1500 HV, it would become very brittle and consequently wouldsnap easily. With the process according to the invention thisdisadvantage is avoided.

As is shown in FIG. 3, which shows a cross-section of a plasma-nitridedlamella, the compound layer is indeed covering the entire surface in aneven manner, assuring sufficient wear resistance on all sides. AlthoughU.S. Pat. No. 647,280 states that with plasma nitriding of intricateshapes it is difficult to achieve an even layer of hardened material(because of which a two-stage process for nitriding is proposed), thepresent manufacturing process does not suffer from this problem. As canbe seen in the cross-section in FIG. 3, an even thickness of thenitrided layer is achieved.

Prior to the nitriding process, the maraging and precipitationhardenable steels must first be hardened by an ageing heat treatment.Optionally this may be combined with the nitriding process as this,according to the present invention, is carried out at the sametemperature. The plasma nitriding process employed here is commonlyknown in the art.

PREFERRED EMBODIMENTS

To better illustrate the present invention two examples are given below,in which stainless maraging steel and austenitic stainless steel,respectively, are used. These examples are strictly non-limitative, asany type of steel with suitable properties may be used.

EXAMPLE 1

Manufacture of a shaver head according to the invention from 1RK91maraging steel.

After manufacture, the cutting element is kept in a pulsed nitridingfurnace at 375° C. for 20 hours in 475 Pa nitrogen gas pressure, duringwhich the nitriding takes place. With an average thickness of thelamella of around 70 μm this results in a compound layer of around 10 to20 μm. As can be seen in the schematic representation in FIG. 6, thediffusion zones just touch. In the case of 1RK91 steel, the hardness oforiginally 500 HV has been increased to 1500 HV on the outside of thecompound layer. Also the Young modulus increases in the compound layerby 23%, rising from 177 GPa to 217 GPa.

EXAMPLE 2

This proceeds in an analogous manner to example 1, but with the use ofAISI 316 austenitic steel. The chosen temperature is 425° C. Theresulting hardness ranges from the original 200 HV in the center core ofthe lamella to 1400 HV on the outside surface.

Thus it is shown that a variety of steels can be hardened by plasmanitriding to obtain a desired hardness of around 1500 HV. In either casethe corrosion resistance was not impaired.

The method according to the invention may obviously also be applied toother devices that are subjected to high wear and corrosive conditions,such as, but not limited to, razors, rotating knives, cutting tools,certain automotive parts, etc.

1. Cutting element as used in an electric shaver, manufactured frommaraging or precipitation-hardenable stainless steel with a surfacehardened by plasma nitriding, characterized in that the cutting elementis hardened by plasma nitriding on all surfaces of the blade, and aplasma nitriding hardened layer consist of a surface top layer of steelsupersaturated with nitrogen and a diffusion layer adjoining the toplayer with a hardness ranging from the hardness of the top layer to thehardness of the steel before hardening by means of plasma nitriding. 2.Cutting element as claimed in claim 1, characterized in that thethickness of the hardened supersaturated top layer ranges from 5 μm to25 μm.
 3. Cutting element according to claim 1, characterized in thatthe thickness of the diffusion layer ranges from 5 μm to 20 μm. 4.Cutting element according to claim 1, characterized in that the hardnessof the hardened supersaturated top layer is at least 1300 HV.
 5. Cuttingelement according to claim 1, characterized in that the cutting elementis designed for use in a shaver of the dry shaver type.
 6. Cuttingelement according to claim 1, characterized in that the cutting elementis designed for use in a shaver of the additive shaver type.
 7. Electricshaver comprising at least one of the cutting elements according toclaim
 1. 8. Method of manufacturing a cutting element, characterized inthat a cutting element is formed of austenitic stainless steel,whereupon the cutting element is hardened on all surfaces by means ofplasma nitriding to a hardness of the top layer of at least 1100 HV. 9.Method according claim 7, characterized in that, after the cuttingelement has been formed from stainless maraging steel orprecipitation-hardenable stainless steel, the cutting element isprecipitationally hardened prior to or simultaneously with the plasmanitriding.